Switch timing from erlang:now/0 -> os:timestamp/0

Performance improvement tested via the 'null' driver, e.g.,

    {operations, [
        {put, 1}
    ]}.
    {mode, max}.
    {concurrent, 100}.
    {duration, 999}.
    {report_interval, 1}.
    {driver, basho_bench_driver_null}.
    {key_generator, {partitioned_sequential_int, 10000000}}.
    {value_generator, {fixed_bin, 1000}}.
Change process priority of basho_bench_stats proc to high

Purpose: to create more regular reporting intervals in
our CSV files.

For all timings, I used the null driver & config found in
the message for commit 304314e89.  Results below are for
four runs each.

Before:
    Runtimes (seconds): 83.4, 87.0, 85.0, 85.6
    wc -l of summary.csv: 6, 9, 7, 9

After:
    Runtimes (seconds): 85.4, 84.8, 88.3, 86.3
    wc -l of summary.csv: 49, 47, 51, 48
Create more regular reporting intervals in our CSV files

NOTE: The type written to both columns 1 & 2 of the CSV
files ("elapsed" and "window") have changed from
integer -> floating point.

For all timings, I used the null driver & config found in
the message for commit 304314e89.  Results below are for
four runs each.

Before:
    Runtimes (seconds): 85.4, 84.8, 88.3, 86.3
    wc -l of summary.csv: 49, 47, 51, 48

After:
    Runtimes (seconds): 85.8, 86.6, 85.0, 85.0
    wc -l of summary.csv: 87, 88, 86, 86
Reduce spamminess of seq_gen_state_dir check
Add config option: disable_sequential_int_progress_report
Fix get(you_have_been_warned) boolean bug-think-o
Check for correct value

The you_have_been_warned key is never set of false, only to true.
Its initial value is undefined.
Merge branch 'slf-null-performance-enhancements'